It has been well known for a long period of time that one of the important attributes of an artificial leg for achieving a natural-looking walking gait is a so called stabilised knee, i.e. a knee which resists flexion when under load, that is when it is bearing at least some of the weight of the amputee. Purely mechanical devices have been produced such as one including a drum supported on radius arms and encircled by a friction brake band as disclosed in British Patent No. 779087, and one including a drum with an internal brake shoe coupled to a radius arm and a toggle link as shown in British Patent No. 1534181. In both of these devices an axial load on the limb produces a small rotation of the radius arm or arms causing the brake band or brake shoe to grip the drum and to resist knee flexion. Indeed, the resistance may be such that the knee is automatically locked if sufficient load is applied. Often such devices are combined with a pneumatic piston and cylinder assembly which applied lower degrees of resistance to flexion and/or extension of the knee to control the motion of the shin during the swing phase.
It is also known to provide resistance to flexion during the stance phase as well as the swing phase by means of a piston and cylinder assembly. One example of such an arrangement is the hydraulic "S-N-S" knee control system manufactured by Mauch Laboratories, Inc. In some situations, however, this system requires the amputee to make a conscious knee-extending movement before flexion can be initiated.